A semiconductor oscillator using a transistor as an amplifier is one of microwave-band oscillators. This semiconductor oscillator is used in frequency bands up to about 1,000 MHz. Since, however, the types of semiconductor devices usable in a frequency band of 2,450 MHz necessary for plasma generation are few and they are expensive, semiconductor oscillators require very high costs.
2,450 MHz oscillators are classified into a klystron oscillator and magnetron oscillator. The klystron oscillator has a high frequency stability and high amplitude controllability. However, a klystron is expensive, and the cost of a power supply for driving the klystron is relatively high, so the whole apparatus including them requires a very high cost.
By contrast, the magnetron oscillator is available at a low cost because magnetrons up to an output power of about 10 kW required for plasma generation are mass-produced. Also, a magnetron driving power supply is simple in arrangement and can be manufactured at a low cost. Accordingly, many magnetron oscillators are used as, e.g., magnetron power supplies for plasma generation.
The characteristics of the magnetron used in the magnetron oscillator will be explained below.
FIG. 12 is a Rieke diagram showing the load characteristics of the magnetron. In this diagram, the relationship between the output power, oscillation frequency, and load impedance obtained by connecting a load to the output terminal of a test launcher attached to the magnetron and changing the impedance of this load is displayed on a Smith chart under the conditions that the output power and frequency are constant. A graph when the output power is constant is indicated by the solid lines, and a graph when the frequency is constant is indicated by the dotted lines.
As shown in FIG. 12, the magnetron changes the output power and oscillation frequency in accordance with the load impedance. A phenomenon in which the oscillation frequency changes in accordance with the load impedance is called a “pulling phenomenon”.
FIG. 13 is a performance chart showing the operation characteristics of the magnetron. This chart shows the changes in anode voltage, output power, and oscillation frequency when a matched load is connected to the output terminal of a test launcher attached to the magnetron, and the anode current is changed while the magnetron is in operation. Graphs 201, 202, and 203 respectively indicate the changes in anode voltage, output power, and oscillation frequency.
As indicated by the graphs 201 to 203 shown in FIG. 13, the anode voltage is substantially constant with respect to the anode current, the output power changes in substantially proportion to the anode current, and the oscillation frequency changes with the anode current although this change is smaller than standard values (the frequency change is 15 MHz, and the rate of change is 0.6%).
Also, when the anode current is changed to change the output power to be equal to or smaller than a certain value, the oscillation mode of the magnetron jumps to cause oscillation at a different frequency, and this makes the oscillation state unstable. This phenomenon is called “moding”. If moding occurs, intermittent oscillation takes place, and the oscillation frequency is no longer constant, thereby causing the inconveniences that, e.g., a matching circuit or the like connected to the load side does not normally operate any longer (see reference 1 (“Techniques of Microwave Plasma” edited by Electric Society Microwave Plasma Investigation Expert Committee, Ohm, Sep. 25, 2003, pp. 240-243)).